NEUR 0010 - Chapter4

Question 1

What is the rising phase of an action potential?

Answer 1

Depolarization, peaking at 40 mV

Question 2

What is the falling phase of an action potential?

Answer 2

Repolarization

Question 3

What is overshoot?

Answer 3

When the inside of the neuron is so depolarized that it’s actually positive

Question 4

What is undershoot?

Answer 4

When repolarization is so intense that the membrane potential is more negative than usual

Question 5

How does an action potential start?

Answer 5

Depolarization: if it reaches the threshold level of depolarization (if inside gets less negative enough), will trigger an action potential

Question 6

How are firing frequency and depolarizing current related?

Answer 6

The higher the magnitude of the depolarizing current, the higher the firing frequency

Question 7

What is the approximate maximum firing frequency of a neuron, and why?

Answer 7

Approximately 1000 Hz, because of the absolute refractory period

Question 8

How does the relative refractory period differ from the absolute refractory period?

Answer 8

Relative refractory period is at the end of the absolute refractory period, and requires a higher depolarization than normal to activate another action potential

Question 9

What ions cause depolarization and repolarization?

Answer 9

Depolarization is caused by the influx of Na+ ions, and repolarization is caused by the efflux of K+ ions

Question 10

What allows the entry of Na+ ions after depolarization to threshold?

Answer 10

The transient increase in g(Na) [sodium conductivity], which helps drives sodium ions in, allowing the real depolarization to set in, which activates the voltage-gated sodium channels

Question 11

How does a voltage-gated sodium channel work, basically?

Answer 11

When depolarized above threshold, the voltage opens the channel; when potential reaches a certain positive value, voltage closes the channel

Question 12

What is the peptide structure of a voltage-gated Na channel?

Answer 12

Long polypeptide with four domains (six subunits each), clumped together to form a pore: domains twist after depolarization to threshold, opening the pore; has pore loops assembled into a selectivity filter that allow Na+ but not K+

Question 13

How does the selectivity filter in the voltage-gated Na channel operate?

Answer 13

Strips most companion water molecules from the Na+ ion : excess water serves as a molecular chaperone, allows the partially-hydrated Na+ to pass through the filter

Question 14

What is the approximate voltage of the threshold in a neuronal membrane?

Answer 14

-40 mV

Question 15

What are the characteristic behaviors of the voltage-gated sodium channel?

Answer 15

Open after depolarization to -40 mV, only for 1 msec, can’t be opened again until membrane potential has been repolarized again to a value below threshold

Question 16

How does tetrodoxin affect sodium channels?

Answer 16

Blocks: binds tightly to a specific site on the outside of the channel

Question 17

What are two toxins that block sodium channels?

Answer 17

TTX and saxitoxin

Question 18

How does bactrachotoxin affect sodium channels?

Answer 18

Causes them to open inappropriately: open at more negative potentials, stay open too long (doesn’t inactivate)

Question 19

What are three toxins that cause sodium channels to open at subthreshold potentials and prevent inactivation?

Answer 19

Batrachotoxin (Colombian frog), veratridine (lilies), and aconitine (buttercups)

Question 20

What is the “delayed rectifier?”

Answer 20

When potassium conductance increases to reset the membrane potential

Question 21

When do voltage-gated K+ channels typically open?

Answer 21

After the membrane’s been depolarized beyond threshold, to try to bring it back down to threshold potential

Question 22

What does it mean that action potentials are orthodromic?

Answer 22

Can only travel in one direction (normally, action potentials are orthodromic)

Question 23

Why does action potential conduction increase with axonal diameter?

Answer 23

Consider the axon like a water hose: if it’s narrow and there are too many pores open, it’ll leak and not conduct well; if it’s wider and has fewer open pores, then it will conduct well

Question 24

How does axonal size affect axon excitability?

Answer 24

Smaller axons require more depolarization to reach threshold, more sensitive to being blocked by local anesthetics

Question 25

How are our neurons effective if they’re not wide enough to prevent “leakage?”

Answer 25

Myelin!

Question 26

Why are nodes of Ranvier useful?

Answer 26

Allow ions to enter the axon to initiate action potentials

Question 27

What kind of channel is concentrated in nodes of Ranvier?

Answer 27

Voltage-gated sodium channels

Question 28

What is a good metaphor for saltatory conduction?

Answer 28

Skipping down the sidewalk

Question 29

Can neurites initiate action potentials?

Answer 29

Generally, NO!

Question 30

What is the spike-initiation zone?

Answer 30

Where the axon can start producing action potentials

Question 31

Where is the spike-initiation zone for CNS vs PNS neurons?

Answer 31

CNS: often at the axon hillock; PNS (sensory neurons); at sensory nerve endings, where depolarization is caused by sensory stimulation